It is also known from U.S. Pat. No. 4,342,733 to use a topping agent selected from the group consisting of polyphosphate compounds and organophosphonic acid compounds in a scrubbing liquor to supplement and enhance the antioxidant activity of sulfite antioxidant agents such as the linear polyethyleneamines and substituted secondary and tertiary aromatic amines.
One problem with the polyethyleneamine sulfite antioxidants employed in the prior art is that the polyethyleneamines are generally volatile to some extent, particularly the lower molecular weight polyethyleneamines such as triethylenetetramine and tetraethylenepentamine. This results in process losses from the sulfur dioxide scrubbing system and can also pose an odor problem. In addition, such amines can be difficult for waste water treatment systems to tolerate, and therefore, disposal of spent scrubbing solution from systems employing the polyethyleneamines is complicated. For example, the lower molecular weight polyethyleneamines are not retained on ultrafiltration membranes.
Another problem with the polyethyleneamines as additives for sulfite oxidation inhibition is that they readily complex with metals, and it is necessary to add additional quantities of the polyethyleneamines to the scrubbing liquor when metals and other cations are present in the scrubbing liquor. Since the oxidation of sulfite to sulfate is trace-metal catalyzed, and suppressed by metal complexing agents as described in Huss, Jr., et al. J. Amer Chem. Soc., v. 100 p 6252 (1978), an explanation for the anti-oxidant properties of such polyethyleneamines is that they complex with any metal ions which may be present in the scrubbing liquor to inhibit such catalysis, and do not function as true antioxidants.
It is known from U.S. Pat. No. 4,530,704 to use a process for selectively removing and recovering sulfur dioxide for a gas stream containing the same by contacting the gas with absorbent (aqueous piperazine, piperazinone or a morpholinone) and thermally regenerating the absorbent (releasing the sulfur dioxide) for reuse in the contacting step.
It is also known from U.S. Pat. No. 4,783,327 to use a method of removing sulfur dioxide from a gas stream containing the same by absorbing the sulfur dioxide in an aqueous solution containing a 4-(2-hydroxyalkyl)-2-piperazinone compound or alkyl or aryl substituted derivative thereof. These compounds provide an absorbent solution which is less subject to loss during the regeneration step and yet effectively remove the sulfur dioxide from the gas stream without interference from other associated gases.
It is known from U.S. Pat. No. 4,324,775 to use a water-immiscible organic solution of a hydrophobic primary, secondary, tertiary or quaternary amine salt as a sulfite transfer agent for contacting an SO.sub.2 -pregnant aqueous alkali SO.sub.2 scrubbing solution to enhance SO.sub.2 scrubbing. The organic solution is subsequently heated to release SO.sub.2 in a stripping step, and then recontacted with the aqueous alkali scrubbing solution to regenerate the aqueous solution for recycle to the scrubbing step wherein gaseous SO.sub.2 is contacted with the regenerated aqueous alkali SO.sub.2 scrubbing solution.
Other sulfur dioxide absorption solvents, buffers and methods are described in various references, such as, for example, U.S. Pat. Nos. 4,387,037; 4,363,791; and 3,873,673.